The primary means of attaching components to a printed circuit board has been to have holes formed within the circuit board through which the leads of the components could be placed and solder applied on the bottom side of the board where the leads egressed. The circuit board would then be passed through a solder wave in which the leads would be soldered to the circuit board.
Another method of attaching components to printed circuit boards is using surface mounting techniques wherein either the component is leadless or its leads are formed so that the component can be freestanding. Surface mounting techniques better automate the attachment of components to printed circuit boards since they improve reliability of attaching components to the circuit board and reduce the amount of area that needs to be used on a printed circuit board. When surface mounting, the solder is typically applied as a room temperature solder paste into which the component's terminals are placed and the board is then heated to a temperature at which the solder paste will reflow as liquid solder, thereby attaching the components to the printed circuit board.
The four typical methods by which reflow heat can be applied to a printed circuit board on a production basis are:
1. Conduction--the bottom surface of the board is heated with the heat passing through the board by conduction and reflowing the solder paste.
2. Vapor phase--a neutral liquid material is vaporized, the printed circuit board is placed in the vapor, the latent heat of the vapor is transferred to the board causing the solder paste to reflow and the vapor in transferring its heat returns to being a liquid for subsequent re-heating.
3. Convection--a heated gas, typically just air, but sometimes nitrogen, is gently directed at the printed circuit board and the solder paste is reflowed.
4. Infrared--radiant infrared energy is directed at the printed circuit board and the solder paste is reflowed.
Of the four different methods of surface mounting components, the infrared reflow method offers the highest volume production capability with respect to the cost of the equipment involved.
The primary disadvantage in using infrared reflow is that when components, such as plastic film capacitors, are subjected to radiant infrared energy at the levels being used to reflow solder on printed circuit boards, the components will absorb excessive amounts of energy to such a degree as to physically damage the components thereby causing the circuit board to work improperly. This has required the components to be mounted in a post assembly operation with the soldering being accomplished by conduction or convection heating.
Another disadvantage of using infrared reflow processing with plastic film capacitors is that the infrared radiant energy will typically heat the plastic film capacitors much faster than the joints that are to be soldered. The major reason for this is the higher absorption rate of the plastic film capacitor as compared with the materials forming the joint such as copper or tin.